The present invention relates to safety switches.
A safety switch may be considered as an emergency electrical shut off switch, and either allows or prevents electricity from passing through it (i.e. it provides a closed circuit or an open circuit). If the safety switch is ‘open’, such that it forms an open circuit, electricity will not pass to an apparatus to which the safety switch is connected.
Safety switches are often used in places where access to machinery is restricted by machine guards which surround the machinery. For example, safety switches are often found in factories that use kinetic machinery powered by electricity. The safety switch may be used to prevent access to a machine via a machine guard when the machinery is in operation. Specifically, power will only be supplied to the machinery when the safety switch is ‘closed’ (i.e. forming a closed circuit), and this is conveniently achieved by the closure of a gate or door incorporated in the machine guard. When the gate is opened, the safety switch is also opened, causing a break in the circuit which prevents electricity being supplied to the machinery (i.e. the machinery cannot run when the gate is opened). Safety switches are well known in the art, and come in a variety of different forms.
One type of safety switch that is used to control access to a machine via a machine guard (or other enclosure) incorporates a reed switch. An electric circuit comprising a reed switch is located, for example, in a fence post of the machine guard. The reed switch is biased to an open position by, for example, a spring. When the reed switch is open there is an open circuit, which prevents electricity being supplied to machinery within the machine guard. A magnet is provided on a door to the machine guard and is positioned such that, when the door to the machine guard is closed, the magnet is adjacent to and in close proximity with the reed switch. Closure of the door brings the magnet into proximity with the reed switch, which causes the reed switch to close. When the reed switch closes, electricity may be supplied to the machinery within the machine guard. If the door is open, the magnet is no longer in close proximity with the reed switch, and the bias applied to the reed switch causes it to open, forming an open circuit. Electricity is then no longer supplied to the machinery.
Many safety switches incorporate reed switches. However, reed switches have a number of disadvantages. For example, the reed switch may become welded closed due to the large amount of current flowing through the reed switch. When the reed switch is welded closed, electricity may be supplied to machinery within the machine guard whether or not the door to the machine guard is open or closed. Thus if the reed switch welds closed, a user may enter the machine guard when the machinery is operating, which is contrary to the purpose of the safety switch.
As described above, a reed switch is opened and closed by bringing a magnet into close proximity with the reed switch. Thus, with the prior art safety switches which incorporate a reed switch, a user can circumvent the safety switch by placing a magnet adjacent to the reed switch to close the reed switch. By placing a magnet adjacent to the reed switch, the reed switch can be closed and electricity can be supplied to the machinery within the machine guard. A user can apply a magnet to the reed switch without closing the door to the machine guard, which means that a user can enter the machine guard while machinery is operating. Again, the purpose of incorporating a safety switch is to avoid such a scenario.
Although the problem of welding is particularly relevant to reed switches, other magnetically operated switches can also become welded closed.
It is thus desired to obviate or mitigate at least one of the above-mentioned disadvantages.